The invention relates to a straddling dowel of the kind wherein the segments thereof press against the sides of the drill hole when the screw is inserted, in that the conically shaped wood screw that is inserted into the screw insertion hole forces the segments to the outside. Straddling dowels of this kind, have a closed foot (bottom) portion, in which the inserted screw can be imbedded by rotation of the screw. Such rotation also results in axial movement of the dowel and causes expansion, also have a buckling. As a result of the axial motion of the dowel the dowel segments that are separated by the longitudinal slits buckle radially to the outside if the dowel shell is unrestrained, i.e., is a hollow block or can spread out behind a panel. Dowels that have this double function have proved to be especially valuable in practice, since it has been possible to use particular types of screws suitable for particular cases. In general, the dowels were configured in such a way that in each case it is possible to use two different diameters of screws, according to whether easy insertion of the screw or great resistance to withdrawal was required in a particular case. However, resistance to withdrawal was reduced considerably if screws of smaller diameter were used.
The invention undertakes the task of improving dowels of the type described in such a manner as to ensure greater adaptability to various requirements and a greater variation of the screw fasteners.
The problem is solved by forming the expansion segments of the dowel from annular, overlapping plates or discs which, on spreading or axial movement of the dowel which results in an increase in diameter move relative to each other over the whole extent of the contact surfaces that run obliquely of conically to the axis of the dowel. By this means both during the dowel body spreading and in the axial movement of the dowel body, the individual plates or discs may move wedgelike on top of each other by sliding on top of each other along their contacting surfaces thus making individual contact between the plates and the sides of the drilling possible or, in the case of hollow blocks, permitting certain segments to penetrate individually into depressions in the sides of the drilling, thereby ensuring particularly good individual anchoring.
It is expedient that the longitudinal slits between the expansion segments be arranged in such a way that they run towards the inside of the dowel, helically to the foot portion of the dowel, and in such a manner that the separation plates so formed rest against the sides of the drilling opposite to the direction of withdrawal. This will ensure that, if a withdrawal force is applied, the plates, having their outer edges against the sides of the drilling, will be pressed more firmly against the sides of the drilling, each individual disc against the part of the sides of the drilling with which it is in contact.
Depending on the manner in which the length of the dowel is altered as a result of axial motion it is possible to bridge larger drillings or use screws of various diameters without reducing the holding power, because although the dowel becomes shorter as a result of the axial movement of the plates on top of each other, it becomes greater in diameter within fairly large limits. The use of a helical division of the dowel segments with a left-hand thread to the slits is a further advantage for the use of screws of various diameters.
An example of the invention will be described on the basis of the drawings, which show the following: